1. Field of the Invention
This invention relates to circuit interrupters and, more particularly, it pertains to AC power circuit breakers in which a gas blast is employed for interrupting performance.
2. Description of the Prior Art
In circuit breakers in which an alternating current arc discharge occurs, the removal of arc energy from a critical zone must keep pace with formation of arc energy so that as the current falls to zero, the remaining arced gas is innocuous to arc reignition. In the post-current-zero period, the critical zone must have a dielectric strength sufficient to prevent breakdown under the condition of a rapidly rising voltage which is a function of the fault condition and circuit.
Experimental findings have led to further clarifying criteria particularly related to the use of relatively low temperature dissociating arc gas media and specifically to the use of sulfur hexafluoride in the class of puffer circuit breakers. In that class piston action is provided, occurring within the pressurized cavity of the breaker so that when the interruption operation is executed, cold gas flows through a nozzle, conditioned by the formed positive pressure gradient along the nozzle's longitudinal axis. The dissociation of the arced-sulfur-hexafluoride gas (SF.sub.6) contributes significantly to the increase of this pressure gradient and ultimately develops a higher gas velocity by known laws of physical chemistry and fluid dynamics. However, when the average enthalpy (kilojoules/gm) of the arced SF.sub.6 exceeds that value corresponding to an average temperature exceeding the fully dissociated gas level, the contribution of the thermalized gas to the development of the pressure gradient is considerably reduced.
Another criterion associated with the foregoing has been the necessity for rectifying a rapid decrease of pressure in the confines of the arc chamber in the time of a descending current side of a half cycle of arc current and the oscillatory nature of longitudinal pressure wave set up in the closed cavity of the circuit breaker. This relates to the objective of establishing a high cold gas pressure in the critical-arc-chamber zone in the post-current-zero period. The dielectric strength of the SF.sub.6 increases with the cold gas pressure.